Method of repairing cavitation damage on a hydraulic turbine

ABSTRACT

The method disclosed involves installation of welding, machining, etc. apparatus on the turbine runner itself, the apparatus extending into close proximity to the turbine draft or discharge tube wall, and then mechanically rotating the turbine runner so that the machining or other apparatus is moved over the wall for conditioning or repairing it. In effect, a vertical boring mill is formed using the runner of the turbine itself as a tool.

United States Patent [191 Goings I 1 Feb. 26, 1974 METHOD OF REPAIRINGCAVITATION DAMAGE ON A HYDRAULIC TURBINE Inventor:

Terre Haute, Ind. 47805 Filed: Jan. 8, 1973 Appl. No.: 322,042

US. Cl. 29/401, 29/l56.8 B, 82/4 Int. Cl 823p 7/00 Field of Search.29/401, 559, 156.8 B; 137/15;

References Cited UNITED STATES PATENTS 3/1951 Julian et a1. 82/4 Xl/1958 Becker 82/4 X 7/1970 Tongurian .[29/401 X 12/1972 Okamoto29/156.8 B X Iden Dean Goings, 4858 N. 13th St,

3,711,927 1/1973 Davidson 29/401 Primary Examiner-Charles W. LanhamAssistant Examiner-D. C. Reiley, III

Attorney, Agent, or Firm-Woodard, Weikart, Emhardt & Naughton [57]ABSTRACT The method disclosed involves installation of welding,machining, etc. apparatus on the turbine runner itself, the apparatusextending into close proximity to the turbine draft or discharge tubewall, and then mechanically rotating the turbine runner so that themachining or other apparatus is moved over the wall for conditioning orrepairing it. In effect, a vertical boring mill is formed using therunner of the turbine itself as a tool.

4 Claims, 4 Drawing Figures PATENIED E L974 3 793 6 98 SHEEF 1 (IF 3BACKGROUND OF THE INVENTION Hydroelectric installations conventionallyconsist of a plurality of electrical generator units each powered by ahydraulic turbine whose runner or rotor shaft is concentric with thevertically mounted generator rotor, water moving by gravity through theturbine draft tube and rotating the turbine runner. Gates control theentry and exit of water to the turbine. In such installations arecurring, serious problem is the large amount of downtimeconventionally required, at intervals, of each generating unit as itsdraft tube liner walls and runner blade are repaired and resurfaced toremedy the pitting and surface deterioration caused by cavitation.

Cavitation occurs when the vacuum normally found at local points withinthe turbine runner and draft tube reach a value at which bubbles ofwater vapor are formed. These bubbles collapse with a violent actionfurther along the flow path where the pressure is slightly increased.The cavitation thus occuring causes a loss in efficiency and results inpitting of the metal and concrete surfaces where the vapor bubblescollapse. The metal surface is broken down so that clean metal isexposed, oxidation then proceeds rapidly and a cavity is produced. Asthe process continues, honeycombing of the material of the runner anddraft tube results. Where cavitation is extreme, runner blades having athickness of one inch may be corroded through in less than a yearsoperation. Experience has shown that pitting occurs most commonly at theback of the runner blades, near where water leaves the runner, and onthe wall of the upper portion of the draft tube. Improvement in thedesign of the turbine and its setting, by providing contours of easycurvature, can minimize the problem but do not eliminate it. The cost ofdesign changes are such, however, that it is usually cheaper to repair,at intervals of for example once a year, the damage done by cavitationthan to prevent all cavitation.

In the past such repair has been accomplished by various techniquesinvolving hand-held arc-air equipment to blast off material at thepitted surface area of the draft tube and runner, and arc-weldingstainless steel or other filling material onto the prepared surfaces.These conventional methods are slow and the down time of thehydroelectric unit being serviced is lengthy. They do not lendthemselves to automatic or semi-automatic tooling.

The method of the present invention involves installing machiningtooling on the runner which extends into operative engagement with thetube wall. A collector ring assembly is installed at the free end of therunner to permit electrical power and pneumatic pressure to beaccessible on the runner itself. A power unit is provided whichmechanically rotates the runner relatively slowly causing the cuttingtool to move over the tube wall. By proper adjustment and observation byan operator riding on the slowly rotating runner, the machining of thedraft tube wall may proceed in semi-automatic fashion, thus preparingthe wall for application, by welding, of stainless steel to the drafttube surface. Welding equipment including welding rod feeding reels arealso installed, by suitable scaffolding, on the runner so that thewelding or metal-filling operation on the tube wall can also proceedsemi-automatically as the runner is slowly rotated. Repair of pittedareas on the runner blades themselves may also proceed during thisoperation by using hand held air-arc tools and hand welding and grindingtools. In effect, vertical boring mill tooling is utilized in preparingthe draft tube surface for the weld metal application and this is madepossible by using the turbine runner itself as the rotating component ofthe tooling. This use of the turbine runner itself as part of thetooling contrasts with the slower, hand tool operation of conventionaltechniques, and results in a thirty to fifty per cent reduction inrepair down time of the generating unit. In addition, the machining andwelding operation done in accord with the method of the presentinvention leaves an improved and more uniform surface on the repaireddraft tube area as compared to the results obtained with conventionaltechniques. Herein the tooling attached to the turbine runner, is attimes, referred to collectively as conditioning devices and, it will beunderstood, this includes machining, welding, grinding and similar drafttube surface conditioning apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view generallyillustrating a typical hydroelectric installation.

FIG. 2 is a sectional, enlarged view of a portion of the structure shownin FIG. 1 undergoing the repair method of the present invention.

FIG. 3 is a perspective view taken generally from the top of the drafttube and in the direction indicated by the lines 3--3 in FIG. 2.

FIG. 4 is a fragmentary, perspective view of a portion of the structureshown in FIG. 2 and taken generally in the direction of the lines 4-4 inFIG. 2.

DESCRIPTION OF THE METHOD Referring initially to FIG. 1, there is showna hydroelectric installation which includes a generator 10 whose rotoris moved by the runner or rotor, indicated generally at 11, of ahydraulic turbine. Gates 14 control the entry of water into the drafttube 16 of the turbine, the blades 17 of the turbine runner beingdisposed within the upper cylindrical portion of the draft tube. Therotational axis of the turbine runner is concentric with the rotationalaxis of the rotor of the generator 10.

Referring to FIG. 2, the generator rotor shaft is indicated at 18 and itis, as previously mentioned, concentric with and rigidly attached to theturbine shaft 19. The generator rotor has an extending portion 31 whichcarries an annular shoe 22 adapted to cooperate with an adjacent member23 which provides for braking the movement of the rotor in the event ofa failure of hearing lubrication or other emergency. This structure isconventional in hydroelectric installations.

The inner surface of the wall of the draft tube 16 and the surfaces ofthe runner blades are subject to erosion and pitting, primarily causedby cavitation as previously explained. In practicing the method of thepresent invention to repair and resurface these components, initiallythe gates are closed and by pumping, where necessary, water is removedfrom the draft tube and runner area of the installation. A temporaryfloor structure 24 is then constructed across the draft tube below therunner hub 11a. A collector ring assembly, shown schematically at 26 isthen attached to the lower end of the runner, the collector ringproviding for transferring electrical power and pneumatic lines(indicated schematically at 27 in FIG. 2) into the runner 11 which, asthe method of the present invention pro ceeds, is rotated while thefloor surface 24, of course, remains stationary. The collector ringassembly 26 is of conventional construction and provides access toelectrical and pneumatic tooling power on the runner ll.

Concurrent with or subsequent to this preparatory step in the method, anexternal, temporary power unit, indicated schematically at 31 isinstalled adjacent the portion 21 which, as previously mentioned,rotates with the generator rotor. The power unit 31 may include anelectric motor 32 which drives V-belts 33, which in turn drive a rolleror traction member 34 which engages the adjacent face of the annulartrack 22 carried by the generator rotor portion 21. A suitable controlpanel 36 may be provided for controlling the drive motor 32 and, it willbe understood, through reduction gearing between the motor and thetraction member 34, energization of the motor will result in arelatively slow rotation of the generator rotor shaft 18 and,consequently, of the runner shaft 19 and runner 11. The speed ofrotation of the runner 11 may be controlled at the control panel 36 butfor normal machining and welding operation on the draft tube 16 will beof the order of three or four revolutions per minute when the draft tubediameter is of the order of 22 feet. This provides a linear speed forthe tips of the runner blades 17 with respect to the stationary drafttube wall of the order of 300 feet per minute.

Either concurrently with or subsequent to the installation of thestructures mentioned above, suitable support members 36 may be welded tothe surface of one of the runner blades 17 and these supports mount avertical member 37 upon which travels a cutting or machining toolholding head 38 which can be adjustably positioned along the verticalmember 37 by means of the adjustment wheel 39. The structure carries aseat 41 for an operator riding the structure and observing the machiningoperation. A machining tool extends from the head 38 into machiningengagement with the adjacent draft tube wall surface.

Scaffolding 42 is temporarily attached to and supported by the runnerblades 17 and supports welding apparatus 43 including the welding rodsupply reel 44. The scaffolding structure carries the vertical member 46and member 46 supports a vertically movable welding head indicatedschematically at 47, the head being adjustably movable vertically alongthe member 46. And an operator seat 48 is provided for movement with thehead 47. As will be evident from FIG. 3, as the turbine runner isrotated slowly the welding head 47 will traverse the curved surface ofthe tube wall 16. As will be evident from FIG. 2, the arrangement of thescaffolding 42 and welding equipment is such that a welder (identifiedat 51 in FIG. 3) can weld pitted areas of the blades 17 as the slowrotation of the runner proceeds.

After the conditioning equipment, such as the cutting tool and thewelding head, have been installed, the drive means 31 may be utilized torotate the turbine runner. The material of the draft tube wall may bemachined off to the desired depth. Weld material such as stainless steelmay then be applied to the wall (a carbon-steel build-up layer may beinitially applied to the wall subsequently covered by the stainlesssteel surface). The tube surface may be again machined to smooth thestainless steel build-up. Severely damaged outer edges of the blades 17may have sections removed and the gaps renewed with stainless steelsections, these operations on the blades 17 being carried on by anoperator with hand held tools while the machining and welding of thetube surface proceeds as the turbine runner is rotated.

The drive unit 31 is operable from the control panel 36 and, preferably,also from the runner itself so that rotation of the runner is under thecontrol of an operator stationed on the runner itself. Emergency brakingof the rotation of the turbine runner can preferably be accomplishedeither from the runner itself or at the control panel 36, the brakingfunction utilizing the conventional air or other brake system built intothe generator structure independently of the normal braking control ofthe generator. In effect, a vertical boring mill is constructed tocondition the turbine draft tube surface with the turbine runner itselfacting as the moving component of the boring mill.

After the conditioning operation directed to the runner blade surfacesand the draft tube surface has been completed, the equipment includingthe machining head 38 and supporting structure 36 is removed, togetherwith the scaffold 42 and the supporting structure for the weldingequipment, also the temporary floor 24 and the drive unit 31. Thegenerator may then be placed back in operation and the equipment movedto a'further hydroelectric generator unit in the installation.

I claim:

1. A method of repairing cavitation-induced pitting in the draft tubewall area and runner blades of a hydraulic turbine component of ahydroelectric installation without requiring any disassembly thereof,said method comprising: initially removing water from the turbine drafttube, subsequently attaching a collector ring assembly to the turbinerunner and through it providing access to electrical and pneumatictooling power on the runner as it is rotated within the draft tube,mounting draft tube wall surface conditioning devices on the runnerwhich are positioned to extend from the runner into proximity with thedraft tube wall, providing an auxiliary drive means suitable formechanically rotating the rotor of the generator component normallydriven by the hydraulic turbine to thereby produce relatively slowrotation of the turbine runner, subsequently employing said drive meansto rotate the runner relatively slowly within the draft tube to causethe surface conditioning devices to traverse the curved surface of thedraft tube wall, and then removing the collector ring assembly thesurface conditioning devices and the auxiliary drive means.

2. A method as claimed in claim 1 in which the runner is rotated at aspeed such that the surface conditioning devices move at a speed of 250to 350 feet per minute.

3. A method as claimed in claim 1 in which welding and finishing ofpitted areas of the runner blades by means of hand-held tools is causedto proceed as the runner is rotated for movement of the surfaceconditioning devices over the draft tube wall surface.

4. A method as claimed in claim 1 but including the additional step,after water removal of installing a temporary floor surface in the drafttube spaced from the turbine runner.

* =l ii

1. A method of repairing cavitation-induced pitting in the draft tubewall area and runner blades of a hydraulic turbine component of ahydroelectric installation without requiring any disassembly thereof,said method comprising: initially removing water from the turbine drafttube, subsequently attaching a collector ring assembly to the turbinerunner and through it providing access to electrical and pneumatictooling power on the runner as it is rotated within the draft tube,mounting draft tube wall surface conditioning devices on the runnerwhich are positioned to extend from the runner into proximity with thedraft tube wall, providing an auxiliary drive means suitable formechanically rotating the rotor of the generator component normallydriven by the hydraulic turbine to thereby produce relatively slowrotation of the turbine runner, subsequently employing said drive meansto rotate the runner relatively slowly within the draft tube to causethe surface conditioning devices to traverse the curved surface of thedraft tube wall, and then removing the collector ring assembly, thesurface conditioning devices and the auxiliary drive means.
 2. A methodas claimed in claim 1 in which the runner is rotated at a speed suchthat the surface conditioning devices move at a speed of 250 to 350 feetper minute.
 3. A method as claimed in claim 1 in which welding andfinishing of pitted areas of the runner blades by means of hand-heldtools is caused to proceed as the runner is rotated for movement of thesurface conditioning devices over the draft tube wall surface.
 4. Amethod as claimed in claim 1 but including the additional step, afterwater removal of installing a temporary floor surface in the draft tubespaced from the turbine runner.